Heat oil in a non stick pan. Fry onions till they turn pink. Add chilli powder and turmeric powder. Fry for a few seconds and add tomatoes. Fry for a minute and add the potatoes, fenugreek leaves and salt. Mix well and cover it with a lid. Keep on a low flame and cook till the potatoes are soft. Stir occasionally. Do not add any water. Open the lid and fry for two minutes. Serve with rotis or rice.